The invention is useful in array scanners for facsimile systems or optical character recognition systems. Such scanners are usually calibrated by adjusting the lamp illumination or the gain of the video signal amplifier relative to a reference level indicated by a predetermined voltage.
One example of an array calibration system is shown in an article in the IBM Technical Disclosure Bulletin (Volume 22, No. 8A, January 1980, pp. 3131-3132) entitled Illumination Regulation For Array Scanner by Bassetti et al. This article describes adjusting the video signal from the scanner to a white reference level defined by a reference voltage. This is accomplished in either of two ways. The scanner scans a white reference. Then the video signal is adjusted by changing the lamp intensity or the sampling interval for the photosensors until the signal equals the reference voltage.
U.S. Pat. No. 4,136,277 entitled Exposure Control System for a Document Copying Machine and issued to J. A. Gerrard is another example of an illumination calibration system. Light reflected from the document being copied is sensed by a single photodetector. The lamp intensity is adjusted until the photodetector signal matches a predetermined signal level defined by a reference voltage.
A little more complex illumination control system is taught in FIGS. 15 and 18 of U.S. Pat. No. 4,153,364 entitled Exposure and Development Control Apparatus for Electrostatic Copying Machine issued to K. Suzuki et al. In FIG. 15, the lamp intensity is regulated relative to a reference voltage. In FIG. 18, the lamp intensity is regulated by a central processing unit that computes the required illumination intensity and generates a signal to control the lamp intensity.
A slightly different illumination calibration system is shown in U.S. Pat. No. 4,133,008 entitled Automatic Illumination Compensation Circuit issued to J. G. Tisue. The Tisue patent teaches adjusting the gain of the analog video amplifier connected to the scanner rather than adjusting the lamp intensity. The adjusted video signal is referenced to a predetermined voltage representative of a white reference video level.
One problem with all of the prior systems is that they calibrate the illumination relative to a fixed reference voltage. They do not maximize the signal output from the scanner or the video amplifier connected to the scanner relative to the maximum signal range of the scanner or the video channel. Further, if per chance the reference voltage happened to correspond to a maximum signal range for the scanner, the reference voltage is not readjusted if the scanner degrades or the photosensor array is replaced. This is a significant problem as scanners are susceptible to aging and contamination, and photosensor arrays vary widely in their signal output levels from array to array.